Color bond surveillance system

ABSTRACT

A color bond surveillance system to ensure that adequate adhesive bonding has been accomplished between two surfaces where such adequate adhesive bonding requires the application of separate coatings to each of the mating surfaces, such as a primer coating in addition to the adhesive coating, wherein one of the items being bonded contains a first color-precursor compound, the coatings separately applied to each of the mating surfaces contain a second color-precursor compound, and the adhesive coating applied over the first coating contains at least a third color-precursor compound. The at least three colorprecursor compounds are co-reactive to produce a desired color different from the colors of the precursors and coating compositions when the coated surfaces are mated, thereby demonstrating proper adhesive installation. A method for ensuring proper application of separate liquid coatings required to accomplish the satisfactory adhesion of two mating surfaces, comprising incorporating into one of the items providing one of the mating surfaces a first color-precursor compound, incorporating a second color-precursor compound into the coatings applied directly to each of the mating surfaces, incorporating at least a third color-precursor compound into the adhesive coating applied over one of the coatings applied directly to one of the mating surfaces, and super-imposing the coated surfaces.

United States Patent [191 Kelsey Nov. 6,1971

[ COLOR BOND SURVEILLANCE SYSTEM David H. Kelsey, Van Nuys, Calif.

[731 Assignees R & G Sloane Manufacturing Company, Inc., Los Angeles,Calif.

[22] Filed: May 14, 1971 [21] Appl. No.: 143,608

[75] Inventor:

[52] 11.8. C1 156/314, ll7/62.1, 138/146,

156/64, 156/309, 156/310, 161/188, 260/141 [51] Int. Cl B32b 7/12, C09j5/04 [58] Field of Search 156/314, 64, 309,

156/310; 138/145, 146, DIG. 1; 117/62.l; 161/188; 260/141 6/1963 France156/64 Primary ExaminerNorman G. Torchin Assistant Examiner.lohn R.Miller Attorn'ey'lipton D. Jennings [57] 7 ABSTRACT A color bondsurveillance system to ensure that adequate adhesive bonding has beenaccomplished between two surfaces where such adequate adhesive bondingrequires the application of separate coatings to each of the matingsurfaces, such as a primer coating in addition to the adhesive coating,wherein one of the items being bonded contains a first color-precursorcompound, the coatings separately applied to each of the matingsurfacescontain a second color-precursor compound, and the adhesivecoating applied over the first coating contains at least a thirdcolor-precursor compound. The at least three color-precursor compoundsare co-reactive to produce a desired color different from the colors ofthe precursors and coating compositions when the coated surfaces aremated, thereby demonstrating proper adhesive installation.

A method for ensuring proper application of separate liquid coatingsrequired to accomplish the satisfactory adhesion of two mating surfaces,comprising incorporating into one of the items providing one of themating surfaces a first color-precursor compound, incorporating a secondcolor-precursor compound into the coatings applied directly to each ofthe mating surfaces, incorporating at least a third color-precursorcompound into the adhesive coating applied over one of the coatingsapplied directly to one of the mating surfaces, and super-imposing thecoated surfaces.

- 33 Claims, N0 Drawings COLOR BOND SURVEILLANCE SYSTEM BACKGROUND OFTHE INVENTION It is frequently necessary to join separate components oritems by adhesive bonding. Such requirements as, for example, inlamination and the assembly of plastic, ceramic, wood, and metaldevices, are well known. The, invention herein is applicable to any suchadhesive bonding wherein the accomplishment of a satisfactory adhesivebond requires the application of a first liquid coating, such as aprimer, directly to each of the surfaces to be bonded, and theapplication of at least one liquid coating of adhesive to one of theprimer-coated surfaces. It is particularly useful in the case of suchadhesive bonding required to form joints between two items or componentsof a system wherein failure of the adhesive bond can result in leakageor other failure of the system. An example of such bonding is thatbetween plastic pipe and plastic pipe fittings in a liquid orgas-carrying system.

Failure or inadequacy of an adhesive bond is frequently caused byover-sight or negligence on the part of the workman, who may omit theapplication of one or more required coatings, such as the primer, orpermit the applied liquid coatings to become excessively dry beforesuccessive coatings or mating of the coated surfaces. such improperadhesive installation generally results immediately or eventually infailure of the adhesive bond. In some cases, an adequate bond may not beachieveddespite proper adhesive installation because of excessivelyloose mating fit of the two items, as for example where the OD of a pipeis too small. It has hitherto been difficult for an inspector todetermine whether, in fact, fit was adequate after the installation hasbeen made.

The invention herein provides a visible surveillance means fordetermining whether an adequate adhesive bond has been achieved by thedevelopment of a desired color in the finished bond;

Kelsey et al. U.S. Patent application, Ser. No. 134,422, discloses anadhesive installation system which comprises a plurality of fluidcoatings, at least one of which is a fluid adhesive composition and atleast two of which are separately applied directly to each of the matingsurfaces to be adhered wherein different color-precursor componentsreactive each with the other are separately incorporated into at leastthe coatings, such as primers, which are applied directly to the matingsurfaces. Additional different colorprecursor components reactive withthe aforementioned color-precursors can be incorporated into additionalrequired coatings, such as adhesive, which are applied over the initialcoatings. When the coated surfaces are superimposed, the differentreactive colorprecursors in the different coatings react to produce thedesired color, thereby providing visual assurance of adequateinstallation.

Although this system is effective and has broad application, it doeshave some potential'disadvantages in some cases. One example is the casewhere the same primer composition is applied to both of the surfaces tobe adhered. The foregoing system requires that the primer be dividedinto two portions, each containing a different color-precursor. Thus theworker is not only required to carry two separate primers but must alsobe careful to distinguish between the two, both in application and inuse of separate brushes or other applicators to avoid contamination ofone primer with the colorprecursor in the other primer. Even where thebasic primer compositions are in fact somewhat different, workmenfrequently tend to use the same applicator with consequent precursorcontamination.

These disadvantages are obviated by incorporating into one of the itemsto be bonded one of the required color-precursor compounds. Thiseliminates the requirement that each of the first coatings, such as theprimer, applied to the mating surfaces contain different color-precursorcompounds with its attendant disadvantages. Additionally, if the firstcoating or primer is properly formulated in terms of color-precursorconcentration, the desired color can be obtained in the finished bondonly if all of the required coatings have been properly applied.

SUMMARY OF THE INVENTION therein a first color-precursor compound, eachof the first coatings applied directly to the two mating surfacescontains a second color-precursor compound, and a fluid adhesive coatingapplied over at least one of the surfaces bearing the first coating,preferably the first-coated surface of the item free from the firstcolorprecursor, contains at least a third color-precursor compound. Theat least three color-precursor compounds are co-reactive to produce adesired color.

When the surfaces bearing the coatings containing the reactivecolor-precursor compounds are placed in superimposed contact, the atleast two reactive colorprecursors separately contained in the fluidcoatings react with each other and with the color precursor in one ofthe items to be bonded to produce the desired color. Development of thedesired color in the finished bond shows that the requisite coatings fora satisfactory adhesive bond have been properly applied during theadhesive installation. 7 e

The aforedescribed color bond surveillance system and process areparticularly useful where at least the exteriorly visible one of theitems being bonded is transparent or translucent so that the colorchange is immediately visible. It is also useful in the bonding ofopaque items as a future means for determining whether failure of thebonded items was caused by the improper adhesive installation or becauseof flaws in the items which were bonded.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The compounds reactive toproduce the desired color can be any color-precursors or intermediateswell known in the chemical, dye, and pigment arts, it being essentialonly that the reactive compounds in the coatings and in one of the itemsbeing bonded co-react to produce the desired color. The number ofco-reactive compounds necessary to produce a product having the desiredcolor may be three or more depending on the particular color-precursorsystem employed. Components such as alkaline buffers, catalysts, and thelike,

which are essential to the primary reaction are considered to bereactive color-precursor compounds for the purpose of this specificationand claims. Preferably the color-precursor reagents selected arecompatible with the composition of the item being bonded and with theparticular primer or adhesive coatings and in addition soluble in thefluid coatings.

The reactive color-precursor compounds can be organic or inorganiccompounds which react to form compounds of different colors.

Particularly useful are the reactive diazo dye intermediates, namelyreactive diazo bases and reactive phenolic developers or couplers whichreact in the presence of an alkaline buffer compound to produce the wellknown azo dyes. Many reactive diazo bases, reactive phenolic couplers,and appropriate buffers are well known and commercially available, whichare soluble in water or organic solvents as required by the particularcoating compositions.

Examples of such reactive diazo bases include: diazotized p-nitraniline,diazotized sulfanilic acid, diazotized naphthionic acid,4-diazo-2,S-diethoxyphenyl morpholine BF,,, 4-diazophenyl morpholine BFp-diazo-ochloro-N ,N -diethyl aniline BF 4-diazo-2 ,5- diethoxyphenylmorpholine /2 ZnCI 4-diazophenyl morpholine y ZnCl The borofluoride saltderivatives are particularly useful because of their solubility inorganic solvent based coatings.

Examples of reactive phenolic coupling agents include: aand B-naphthol,naphtholsulfonic acids, aminonaphtholsulfonic acids,2,3-dihydroxy-naphtholsulfonic acids, 2,3-dihydroxy-naphthalene, 3,5-resorcylic acid ethanolamide, 2,4-resorcylic acid ethanolamide,4-bromo-3,5-resorcylic acid.

Examples of alkaline buffers, which assist in the diazo base-phenoliccoupler reactive system to produce the desired color, include organicmono-, di-, or tertiaryalkylamines, e.g. butyl amine, diisopropylamine,dimethylsoyamine, triethylamine, tributylamine, dimethyl dodecylamine,and the like.

The concentration of the reactive color-precursor compounds incorporatedinto the item to be bonded and into thecoating compositions will varywith the particular compositions and precursors and requires onlyroutine experimentation for the determination of optimum concentrationsfor a particular application. Care, however, must be exercised that theconcentration of the color-precursor compound in the first coatingcomposition applied directly to each or both of the mating surfaces beat least 50 percent of but substantially less than that required toproduce the desired color with the at least two other color-precursorcompounds available in the system. This is essential to ensure that boththe first coatings applied to each of the mating surfaces providetogether a sum total of the same color-precursor compound adequate toreact with the other at least two color precursors available in thesystem to produce the desired color and that a first coating appliedonly to one of the surfaces does not .provide an adequate concentrationto produce the desired color. The maximum concentration of the colorprecursor compound in the first coating permissible to avoid developmentof the desired color after mating of the surfaces if one of the firstcoatings is not applied to one of the surfaces is largely determined bythe particular reactive color-precursors employed. In general, up toabout 75 percent of the concentration required to react with the otherat least two reactive color precursors available for reaction in thesystem is adequate. It should be noted that even with the reduced totalconcentration in a first coating applied only to one surface, some colormay develop. However it develops more slowly and is generally of a huedifferent from that of the desired color which develops with the totalconcentration provided by both first coatings. it should be noted thatthe color-precursor contained in the item and available to the systemfor reaction is limited to precursor at or adjacent to the surface ofthe item. The precursor at or adjacent to the surface is made availableby contact of the item surface with the first coating. It is preferablyof a type which is soluble in the liquid first coating. It will beunderstood that the term available as used in this specification andclaims in connection with the precursors means those amounts of theprecursors that are actually available for reaction after application ofthe coatings. Since adhesive installations for given uses are generallystandardized routines, determination of specific concentrations of eachprecursor to be incorporated into the item and each of the requiredcoatings to achieve the requisite concentrations during installationrequires only routine experimentation for any given use.

The adhesive bonding system may be any conventional system whichrequires at least three coating applications for satisfactory bonding oftwo surfaces. Such systems are well known in the adhesive bonding artand, as is also well known, vary with the nature and composition of thesurfaces being bonded. In many cases, both of the surfaces to be bondedrequire a conditioning treatment with a primer coating prior toapplication of the adhesive coating. The primer composition employedwhere both surfaces require conditioning may be the same or different,again depending on the nature and composition of the two surfaces to bebonded.

The term adhesive as used herein includes any liquid coating compositionwhich is capable of forming a bond or weld between two superimposedsurfaces. It includes natural and synthetic adhesives such as animal andcasein glues, epoxies, phenol-aldehydes, ureaaldehydes, polyesters,furanes, asphaltic compounds, sodium silicates, polyvinyl acetate, andthe like. They also include solvent cements, such as volatile solventsolutions of polyvinyl chloride, acrylonitrile-butadienestyrenepolymers, cellulose acetate, and the like, which act as welds forsolvated plastic surfaces.

The adhesives may be solutions, emulsions, or slurries in an aqueous orvolatile organic liquid vehicle, such as ethanol, methanol, methyl ethylketone, acetone, tetrahydrofurane, and the like.

The primer can be any conventionally employed liquid composition whichprepares the surface of the articles to be bonded for satisfactoryadhesive bonding. It may be a volatile organic cleaning solvent orvolatile organic solvent which softens or dissolves the surface. it mayalso be an aqueous or organic solvent solution containing a componentwhich improves the bond between the adhesive and the surface to bebonded.

The color-precursor compound in one of the two items being bonded can beincorporated in any conventional manner prior to forming the item. Inthe case of a piastic item, for example, the compound can be mixed withthe plastic formulation or molding composition prior to molding andcure. The other at least two color-precursor compounds are separatelyadded to and admixed with the first or primer liquid coating or coatingsand the adhesive second coating. If more than three color-precursorintermediates are required for development of the desired color, theadditional intermediate can be added to the item being bonded or to thefirst or second coating containing one of the other precursors, with duecare as to compatibility, or it can be added to an additional coating.

When all of the coatings containing the reactive color-precursors areproperly and successively applied to the surface of the item containingone of the required color precursor intermediates and to the surface ofthe item free from precursor, and the coated surfaces are brought intosuperimposed contact, the reactive colorprecursor components are broughtinto contact in adjacent coatings and the adjacent precursor-containingitem or migrate through intermediate coatings to form the desired color.Omission of any required coating will not produce the desired colorchange. Optimum bonding generally requires that at least the adhesiveand preferably also the primer coatings be in fluid or semifluid stateat the time of successive applications and mating of the surfaces. Thisalso facilitates reaction and migration of the reactive color-precursorintermediates. Thus the color bond surveillance system described hereinalso provides a means for determining whether the coatings were in therequired state during successive coating applications and superimposedcontact of the coated surfaces.

Development of the desired color only after completion of adhesive bondinstallation is ensured by limiting color precursor to one of the itemsand limiting the concentration of color precursor in the first coatingsapplied directly to the two surfaces being bonded to substantially lessthan that required for desired color development. If, for example, theworkman omits the first or primer coating, one of the requiredcolordeveloping intermediates is omitted. If ,the first or primercoating is applied to one but not the other of the two surfaces, theconcentration of color-precursor intermediate in the one first coatingapplied is inadequate for development of the desired color, given theconcentration of color precursor in one of the two items being bonded.Although some color may develop it will be slow and will not be of thedesired hue.

Preferably the second adhesive coating containing the at least thirdcolor-precursor is applied over the first coating on the surface of theitem free from colorprecursor since this will minimize any colordevelopment prior to contacting of the coated surfaces. Application overthe first coating on the surface of the item containing color-precursorwill result in some color development. However, since the concentrationof precursor in the first coating is inadequate for full colordevelopment, the desired color hue develops only after completion of theinstallation.

The color bond surveillance system is particularly useful when employedin the adhesive bonding of items at least the outer one of which istransparent or translucent, since the color change showing the adequacyof the adhesive installation is immediately visible. it is, however,also useful in the'bonding of opaque surfaces. Where the adhesivebonding extends to an edge or edges of the bonded surfaces, the colorchange (or absence of color change) is visible at such points. It alsoassists in determining cause of failure of a bonded opaque joint afterdisassembly.

The system has particular application in the solvent cementing ofplastic pipe and fittings. Such pipe and fittings, when made of asolvent-soluble plastic, such as polyvinyl chloride oracrylonitrile-butadiene-styrene polymer, are generally joined by meansof a welding cement comprising a solution of the particular pipe orfitting polymer in a volatile solvent which dissolves the joindersurfaces of the pipe and/or fittings. After mating of the solvent-cementcoated ends of the pipe and- /or fittings, the solvent evaporatesleaving a solid, fused joint. I

In many cases and very particularly in the case of polyvinyl chloridepipe and fittings, the joinder surfaces require primer treatment with anappropriate solvent or mixture of solvents to soften and dissolve themprior to application of the solvent cement. The primer coated surfacemust also be still wet or fluid at the time of application of the cementcoating and the cement coating must still be wet or fluid at the time ofmating contact of the pipe and/or fitting. Selection of reactivecolor-precursor compounds which react substantially only while insolvent solution provides a monitoring system as to condition of thecoatings during installation, since little or no color will develop ifthe coatings are permitted to dry excessively. Failure by the installerto apply either or both of the primer coatings or to follow instructionsas to the condition of the applied coatings, can result in failure ofthe bonded joint and leakage in the pipe system.

In some cases, despite proper application of the required coatings, anadequate adhesive bond may not be achieved because the OD of the pipefor example may be too small to achieve a tight fit with the fittingsocket and, therefore, results in inadequate contact of the appliedcoatings. Such inadequate coating contact results in incompletecolor-precursor reaction and, thereby makes inadequacy of theinstallation for such reason quickly visible to the inspector.

The color bond surveillance system provides a rapid and effectivevisible inspection means for determining adequacy of the bonded joint,particularly if the outer mating member, eg the pipe fitting, is made ofclear transparent plastic, e.g. transparent polyvinyl chloride. Propercolor development may also be visible through a translucent plastic. Incase the outer joinder surface is opaque, proper use of all of therequired coatings may be monitored by the color of the cement whichgenerally extrudes beyond the end of the fitting socket.

Preferably the color-precursor is incorporated into the plastic pipefitting rather than the pipe to which it is bonded since theconsiderably smaller size of the fitting reduces the cost ofcolor-precursor requirement as well as for other reasons of convenience.

The co-reactive diazo base-phenolic coupler-buffer dye system isparticularly useful in the pipe and fitting application because of itscompatibility with such systems. The phenolic coupler is preferablyincorporated into the fitting because of its heat stability under hightemperature injection molding temperatures. The diazo base if perferablyincorporated into the first or primer coating which is applied directlyto the surface of the fitting and the pipe. Generally, the same primer,containing the diazo base in the reduced concentration as aforedescribedrelative to the concentration of the phenolic coupler in the fitting,can be applied to the mating fitting and pipe surfaces. In someapplications primers of somewhat different solvent compositions may bedesirable. In such case the same diazo base is dissolved in thedifferent primers so that accidental dipping of the applicator intoeither applicator has no deleterious effect on the surveillance system.The alkaline buffer is preferably admixed with the secondadhesivecoating.

EXAMPLES Standard clear polyvinyl chloride resin blends were mixed in ablender with 0.223 percent by weight of the following phenolic couplers:

A. 2,3-Dihydroxy naphthalene B. 2,4-Resorcylic acid ethanolamide C.4-Bromo-3,5-resorcylic acid Clear transparent fittings were injectionmolded from the above blends in sizes ranging from three-fourths inch to3 inches.

A primer for application to both fitting and pipe was prepared bydissolving 0.2 percent by weight of 4-diazo-2,5-diethoxyphenylmorpholine BE, in a solvent mixture of tetrahydrofurane andcyclohexanone.

Commercial cements comprising a solution of polyvinyl chloride intetrahydrofurane and cyclohexanone were admixed with the followingalkaline buffers:

A. n-Butylamine 0.1 percent by weight B. Triethylamine 0.3 percent byweight I. The mating surface of a 2 inch deep socket fitting molded fromresin blend B and the mating end surface of a length ofpolyvinylchloride pipe were liberally coated with the primer. Cement Awas then applied over the still fluid primer coating on the pipe. Thepipe and fitting were then mated. After joinder of the coated surfaces,immediate reaction of the color-precursor compounds developed to a deepblue color visible through the clear plastic fitting.

2. The mating surface of a 3 inch deep socket ell fitting molded fromresin blend A and the mating end surface of a length of polyvinylchloride pipe were liberally coated with the primer. Cement B was thenapplied over the still fluid primer coating on the pipe. The pipe andfitting were then mated. After joinder of the coated surfaces animmediate color change developed and within seconds the color haddeveloped to the desired deep blue hue. Similar results were obtainedwith fittings and pipes in sizes ranging from three-fourths inch to 3inch.

3. The same procedure was followed as in Example 2 except that the pipeonly was primed. When the pipe and unprimed fitting were joined, noreaction became visible for several hours. After 24 hours a mottledlight purple color had developed.

4. The same procedure was followed as in Example 2 except that thefitting only was primed and the cement was applied to the umprimed pipe.Color was slow to develop and when developed was a distinct purplerather than the desired deep blue.

Although this invention has been described with reference toillustrative embodiments thereof, it will be apparent to those skilledin the art that the principles of this invention can be embodied inother forms but within the scope of the claims.

I claim:

l. in a method for adhesively bonding two surfaces of two items whereineffective bonding requires the separate application of a first liquidcoating directly to each of said surfaces and a second liquid coatingover one of the first directly-applied coatings, at least the secondliquid coating being a liquid adhesive composition, the improvementcomprising:

1. incorporating a first color-precursor compound into at least one itemhaving one of said two surfaces;

2. incorporating a second color-precursor compound into each of thefirst directly-applied coatings; and

3. incorporating at least a third color-precursor compound into thesecond liquid coating;

said first, second, and at least third color-precursor compounds beingreactive to produce a desired color and said second color-precursorcompound being incorporated into each of the first directly-appliedcoatings in an amount of at least about 50 percent, but substantiallyless than, that required to react with the other availablecolor-precursor compounds to produce the desired color.

2. The method of claim 1 in which the first colorprecursor compound is aphenolic coupler, the second color-precursor compound is a diazo base,and the third color-precursor compound is an alkaline buffer.

3. The method of claim 2 in which the alkaline buffer is an alkyl amine.

4. The method of claim 1 in which the two items are respectively plasticfitting and plastic pipe and the first color-precursor is incorporatedinto the plastic fitting.

5. The method of claim 2 in which the two items are respectively plasticfitting and plastic pipe and the first color-precursor is incorporatedinto the plastic fitting.

6. The method of claim 3 in which the two items are respectively plasticfitting and plastic pipe and the first color-precursor is incorporatedinto the plastic fitting.

7. The method of claim 6 in which the alkyl amine is a primary amine.

8. The method of claim 6 in which the alkyl amine is a tertiary amine.

9. The method of claim 4 in which the plastic fitting is made oftransparent polyvinyl chloride.

10. The method of claim 5 in which the plastic fitting is made oftransparent polyvinyl chloride.

11. The method of claim 6 in which the plastic fitting is made oftransparent polyvinyl chloride.

12. In a method for adhesively bonding a first surface of a first itemto a second surface of a second item wherein effective bonding requiresthe separate application of a first liquid coating directly to each ofsaid surfaces and a second liquid coating over one of the firstdirectly-applied coatings, at least the second liquid coating being aliquid adhesive composition, the improvement comprising:

l. applying to said first surface of said first item, which containsdispersed therein a first colorprecursor, a first liquid coatingcontaining a second color-precursor compound;

2. applying to said second surface of said second item, which is freefrom said first color-precursor, a first liquid coating containing saidsecond colorprecursor compound;

3. applying over said first coating on at least one of said surfaces aliquid adhesive coating containing at least a third color-precursorcompound, said first, second, and at least third color-precursorcompounds being reactive to produce a desired color, and said secondcolor-precursor compound being present in each of the first liquidcoatings applied to said first and second surfaces in an amount of atleast about 50 percent, but substantially less than, that required toreact with the other available color-precursor compounds to produce thedesired color; and I 4. superimposing said surfaces bearing saidcoatings.

13. The method of claim 12 in which the liquid adhesive coatingcontaining the at least third colorprecursor compound is applied overthe first coating on said second surface.

14. The method of claim 13 in which the first colorprecursor compound isa phenolic coupler, the second color-precursor compound is a diazo base,and the third color-precursor compound is an alkaline buffer.

15. The method of claim 14 in which the alkaline buffer is an alkylamine.

16. The method of claim 13 in which the first item is a plastic fittingand the second item is plastic pipe.

17. The method of claim 14 in which the first item is a plastic fittingand the second item is plastic pipe.

18. The method of claim 15 in which the first item is a plastic fittingand the second item is plastic pipe.

19. The method of claim 16 in which the plastic fitting is made oftransparent polyvinyl chloride.

20. The method of claim 17 in which the plastic fitting is made oftransparent polyvinyl chloride.

21. The method of claim 18 in which the plastic fitting is made oftransparent polyvinyl chloride.

22. The method of claim 21 in which the alkyl amine is a primary amine.

23. The method of claim 21 in which the alkyl amine is a tertiary amine.

24. In a method for adhesively bonding a first surface of a first itemto a second surface of a second item wherein effective bonding requiresthe separate application of first liquid coatings to each of saidsurfaces and a second liquid coating over one of said first liquidcoatings, at least the second liquid coating being a liquid adhesivecomposition, the improvement comprisl. applying to said first surface ofsaid first item,

which contains dispersed therein a first colorprecursor, a first liquidcoating containing a second color-precursor compound;

2. applying to said second surface of said second item, which is freefrom said first color-precursor, a first liquid coating containing saidsecond colorprecursor compound;

3. applying over said first coating on said second surface, a liquidadhesive coating containing at least a third color-precursor compound,said first, second, and at least third color-precursor compounds beingreactive to produce a desired color;

4. superimposing said surfaces bearing said coatings.

25. The method of claim 24 in which the first liquid coating applied tothe first and second surfaces each contain the second color-precursorcompound in an amount of at least about 50 percent, but substantiallyless than, that required to react with the other availablecolor-precursor compounds to produce the desired color.

26. The method of claim 24 in which the first colorprecursor compound isa phenolic coupler, the second color-precursor compound is a diazo base,and the third color-precursor compound is an alkaline buffer.

27. The method of claim 26 in which the alkaline buffer is an alkylamine.

28. The method of claim 24 in which the first item is a plastic fittingand the second item is plastic pipe.

29. The method of claim 26 in which the first item is a plastic fittingand the second item is plastic pipe.

30. The method of claim 28 in which the plastic fitting is made oftransparent polyvinyl chloride.

31. The method of claim 27 in which the alkyl amine is a primary amine.

32. The method of claim 27 in which the alkyl amine is a tertiary amine.

33. The method of claim 24 in which said first liquid coatings areprimer coatings.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,770,547 Dated November 6, 1973 lnvent fl David H. Kelsey It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby correctedas shown below:

Page l, at "'[45]" change the date of issuance from "Nov. 6, 1971" toNov. 6, l973-.

Signed and sealed this 9th day of April 197a.

(SEAL) Attest:

EDWARD M.FLETCHIE.R,JR. C. MARSHALL DANN Attesting Officer ICommissioner of Patents FORM PO-10 0 (10- USCOMM-DC 60376-P69 7 U.S.GOVERNMENT PRINTING OFFICE: I959 O-JOG-JJL

2. applying to said second surface of said second item, which is freefrom said first color-precursor, a first liquid coating containing saidsecond color-precursor compound;
 2. The method of claim 1 in which thefirst color-precursor compound is a phenolic coupler, the secondcolor-precursor compound is a diazo base, and the third color-precursorcompound is an alkaline buffer.
 2. incorporating a secondcolor-precursor compound into each of the first directly-appliedcoatings; and
 2. applying to said second surface of said second item,which is free from said first color-precursor, a first liquid coatingcontaining said second color-precursor compound;
 3. applying over saidfirst coating on said second surface, a liquid adhesive coatingcontaining at least a third color-precursor compound, said first,secOnd, and at least third color-precursor compounds being reactive toproduce a desired color;
 3. The method of claim 2 in which the alkalinebuffer is an alkyl amine.
 3. incorporating at least a thirdcolor-precursor compound into the second liquid coating; said first,second, and at least third color-precursor compounds being reactive toproduce a desired color and said second color-precursor compound beingincorporated into each of the first directly-applied coatings in anamount of at least about 50 percent, but substantially less than, thatrequired to react with the other available color-precursor compounds toproduce the desired color.
 3. applying over said first coating on atleast one of said surfaces a liquid adhesive coating containing at leasta third color-precursor compound, said first, second, and at least thirdcolor-precursor compounds being reactive to produce a desired color, andsaid second color-precursor compound being present in each of the firstliquid coatings applied to said first and second surfaces in an amountof at least about 50 percent, but substantially less than, that requiredto react with the other available color-precursor compounds to producethe desired color; and
 4. superimposing said surfaces bearing saidcoatings.
 4. The method of claim 1 in which the two items arerespectively plastic fitting and plastic pipe and the firstcolor-precursor is incorporated into the plastic fitting. 4.superimposing said surfaces bearing said coatings.
 5. The method ofclaim 2 in which the two items are respectively plastic fitting andplastic pipe and the first color-precursor is incorporated into theplastic fitting.
 6. The method of claim 3 in which the two items arerespectively plastic fitting and plastic pipe and the firstcolor-precursor is incorporated into the plastic fitting.
 7. The methodof claim 6 in which the aLkyl amine is a primary amine.
 8. The method ofclaim 6 in which the alkyl amine is a tertiary amine.
 9. The method ofclaim 4 in which the plastic fitting is made of transparent polyvinylchloride.
 10. The method of claim 5 in which the plastic fitting is madeof transparent polyvinyl chloride.
 11. The method of claim 6 in whichthe plastic fitting is made of transparent polyvinyl chloride.
 12. In amethod for adhesively bonding a first surface of a first item to asecond surface of a second item wherein effective bonding requires theseparate application of a first liquid coating directly to each of saidsurfaces and a second liquid coating over one of the firstdirectly-applied coatings, at least the second liquid coating being aliquid adhesive composition, the improvement comprising:
 13. The methodof claim 12 in which the liquid adhesive coating containing the at leastthird color-precursor compound is applied over the first coating on saidsecond surface.
 14. The method of claim 13 in which the firstcolor-precursor compound is a phenolic coupler, the secondcolor-precursor compound is a diazo base, and the third color-precursorcompound is an alkaline buffer.
 15. The method of claim 14 in which thealkaline buffer is an alkyl amine.
 16. The method of claim 13 in whichthe first item is a plastic fitting and the second item is plastic pipe.17. The method of claim 14 in which the first item is a plastic fittingand the second item is plastic pipe.
 18. The method of claim 15 in whichthe first item is a plastic fitting and the second item is plastic pipe.19. The method of claim 16 in which the plastic fitting is made oftransparent polyvinyl chloride.
 20. The method of claim 17 in which theplastic fitting is made of transparent polyvinyl chloride.
 21. Themethod of claim 18 in which the plastic fitting is made of transparentpolyvinyl chloride.
 22. The method of claim 21 in which the alkyl amineis a primary amine.
 23. The method of claim 21 in which the alkyl amineis a tertiary amine.
 24. In a method for adhesively bonding a firstsurface of a first item to a second surface of a second item whereineffective bonding requires the separate application of first liquidcoatings to each of said surfaces and a second liquid coating over oneof said first liquid coatings, at least the second liquid coating beinga liquid adhesive composition, the improvement comprising:
 25. Themethod of claim 24 in which the first liquid coating applied to thefirst and second surfaces each contain the second color-precursorcompound in an amount of at least about 50 percent, but substantiallyless than, that required to react with the other availablecolor-precursor compounds to produce the desired color.
 26. The methodof claim 24 in which the first color-precursor compound is a phenoliccoupler, the second color-precursor compound is a diazo base, and thethird color-precursor compound is an alkaline buffer.
 27. The method ofclaim 26 in which the alkaline buffer is an alkyl amine.
 28. The methodof claim 24 in which the first item is a plastic fitting and the seconditem is plastic pipe.
 29. The method of claim 26 in which the first itemis a plastic fitting and the second item is plastic pipe.
 30. The methodof claim 28 in which the plastic fitting is made of transparentpolyvinyl chloride.
 31. The method of claim 27 in which the alkyl amineis a primary amine.
 32. The method of claim 27 in which the alkyl amineis a tertiary amine.
 33. The method of claim 24 in which said firstliquid coatings are primer coatings.